It is known that the temperatures of combustion gases in gas turbine engines, such as aircraft engines, during operation are considerably above the temperatures of the metal parts of the engine which are in contact with these gases. Operation of these engines at gas temperatures that are above the metal part temperatures is a well established art, and depends on supplying cooling gas to the outer surfaces of the metal parts through various methods. The metal parts of these engines that are particularly subject to high temperatures, and thus require particular attention with respect to cooling, are the blades and vanes used to direct the flow of the hot gases
For example, with regard to the metal blades and vanes employed in aircraft engines, some cooling is achieved by convection by providing passages for flow of a cooling gas internally within the blades so that heat may be removed from the metal structure of the blade by the cooling gases. Such blades are essentially hollow core blades which may have a shell or plural shells of structural metal of intricate design surrounding equally intricate sets of cooling passages within the hollow core blade. Fine internal orifices have also been devised to direct this circulating cooling gas directly against certain inner surfaces of the shell to obtain cooling of the inner surface by impingement of the cooling gas against the surface, a process known as impingement cooling. In addition, an array of fine holes extending from the hollow core through the blade shell can provide for bleeding cooling air through the blade shell to the outer surface where a film of such air can protect the blade from direct contact with the hot gases passing through the engines, a process known as film cooling.
Using combinations of these cooling techniques, the maximum metal surface temperature of a blade can be maintained at about 1,150.degree. C. while the blade is in an environment of hot gases having temperatures of up to 1,650.degree. C.
As is well-known, the operating efficiency of gas turbines and accordingly of jet engines is related to the operating temperature of the engine. To achieve higher operating efficiencies, operation of the engine at higher temperatures is desirable. For engines operating at temperatures up to 2,000.degree. C., it is expected that the metal temperatures can be maintained almost at present levels with current cooling techniques by using a combination of improved cooling designs and thermal barrier coatings. Thermal barrier coatings are well-known ceramic coatings such as yttria stabilized zirconia that are applied to the external surface of metal parts within engines to impede the transfer of heat from hot combustion gases to the metal parts. However, even with the use of advanced gas cooling designs and thermal barrier coatings, it is also desirable to decrease the requirement for cooling gases, because reducing the demand for cooling gases is also well-known to improve overall engine operating efficiency. One way to achieve such a reduction is to improve the cooling of the metal parts immediately adjacent to their outer surfaces.
Another way in which the increased use of cooling air can be avoided, or cooling air requirements can be reduced, is by providing metal parts that are capable of operating above the maximum use temperature of 1,150.degree. C. The provision of metal parts capable of operating at temperatures beyond 1,150.degree. C. would allow either relaxation of cooling requirement or the reduction or elimination of the dependence on the thermal barrier coatings, or both.
It is also well-known that the operating efficiency of gas turbine engines may be improved by reducing the total weight of the metal parts utilized. Currently, because of the required intricate internal cooling passages within metal parts such as blades and vanes, particularly near their outer surfaces, and the fragile nature of the ceramic cores used to define these passages during formation, it is necessary to utilize large tolerances that allow for the possibility of core shifting. The use of materials and processes that would simplify the design requirements for these internal passages would permit the amount of material used in each metal part to be reduced. Also, the use of materials that are less dense would achieve weight reductions for each metal part. Small savings can be significant because of the large number of these metal parts that are utilized in a typical engine.
Reducing the internal complexity of the metal parts by reducing the number of intricate passageways that must be formed by casting would also improve casting yields and provide an added benefit.
Therefore, it is desirable to define airfoils and materials for their manufacture that have improved cooling capability, higher operating temperatures, more castable geometries and reduced weight as compared to present airfoils.